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species. À determination-table of Wille (1903) pointed 
out that I had to do with Chlamydomonas variabilis 
Dangeard. The description, which Dangeard gives of 
it (1898, page 135), and also his picture confirm this. 
The description and picture which Jacobsen gives of 
Chlamydomonas variabilis agreed less with it. Therefore 
it is well perhaps to give a short description of the Alga. 
The swarmspores have a size up to 20 v, as a rule 
the cell is cylindric-ovoid. There are two cilia, which 
spring up at the base of a distinct wart. The eye-spot is 
disciform, and lies in the middle. Generally the chloroplast 
fils the whole cell. There are often many big granules 
of starch. À pyrenoid is not to be seen. Division stadia 
were often found. I did not find more than four 
daughtercells. The first division takes place transversely. 
Occasionally I found capulation stadia. 
$ 2. Some remarks on the nature of the reactions, 
which we observe with living beings. 
We will now speak about the various reactions, which 
Chlamydomonas variabilis can exercise. Especially we 
will pay attention to those reactions which were of great 
importance to my further experiments. Thus we are going 
to speak of the phototaxis, the geotaxis, the thigmotaxis, 
the chemotaxis and the aërotaxis. 
Before beginning with this, however, it is necessary 
that we get a clear idea of the nature of these reactions. 
We might have to do with physical, chemical and with 
physiological processes. Among the physiological phenomena 
the reactions caused by stimulation take up a particular place. 
These differences are not of a great value. In the first 
place it is impossible to draw a distinct line of demarca- 
